Chair back



FIG. 1 is a top, front and right perspective view of a chair backshowing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top plan view thereof; and,

FIG. 7 is a bottom plan view thereof.

The broken lines in the figures illustrate portions of the chair backthat form no part of the claimed design.

CLAIM The ornamental design for a chair back, as shown and described.